Advances in Fatigue Characterization of Nitinol

Lifetime prediction of components that are subjected to cyclic mechanical motion is critical for the design and optimization of devices manufactured from Nitinol. The current Nitinol market is dominated by medical devices, many of which are permanently implanted and experience tens to hundreds of millions of in vivo cycles, and whose safety and durability may be measured by their fatigue and fracture resistance. Descriptions of mechanical fatigue on a microscopic, and even macroscopic, level for these Nitinol-based medical devices are complicated and remain incomplete even today. The source of this complication is primarily due to the ramification of the thermo-mechanical processing history on the mechanical properties, the phases in Nitinol (including austenite, martensite, R-phase, oxide and carbide inclusions, as well as Ni-rich and Ti-rich precipitates), and the loading history dependence of Nitinol.   

Studies on the fatigue properties of Nitinol date back to the 1960s. Such early investigations, however, were performed on materials that lacked the reproducibility compared to modern melting and processing practices.  Therefore, utilizing data from those early studies that is applicable to today’s Nitinol components is difficult.  Similarly though, even contemporary fatigue investigations also suffer from some inconsistencies due to variations in processing, material selection, and loading history.  This variability in the reported data leads to challenges when engineers attempt to formulate a fundamental fatigue understanding from the unified body of fatigue knowledge.

With a system as complex and non-uniform as the human body, not to mention the hundreds of different Nitinol component geometries, it is unsurprising that there exist such a wide variety of testing techniques, each with their own benefits and limitations. In this presentation we will explore the evolution of fatigue testing techniques and the applicability in the determination of fundamental Nitinol material fatigue resistance versus component-specific durability.  Specific test techniques that will be reviewed and scrutinized are: wire rotary bend, surrogate (e.g. diamond-shaped) axial tension or compression, S- or Z-shaped wire forms in bending, radial pulsatile, torsion, and multi-axis deformation.   Furthermore, we will discuss rarely-use, but potentially important fatigue analysis techniques that we as a community should consider for future data interpretation (e.g. energy-based analysis, probabilistic fatigue computation).